Coal, a natural resource, is commonly burned in coal-fired power plants to convert the energy in the coal to other forms of energy such as electricity and heat. Often, such coal-fired power plants include two or more coal-fired combustion Electrical Generating Units “EGUs” or “Units” used to convert coal energy to electricity. Each of these EGUs typically includes a solid coal feed, an air heater, a boiler, an Electrostatic Precipitator (ESP), a wet Flue Gas Desulfurization (wFGD) with a liquid reaction tank, and a chimney stack.
Coal contains contaminants such as mercury and sulfur at variable concentrations, depending on the type of coal deposit and mine source. When coal is burned, flue gas is a natural by-product and the sulfur (S) and mercury (Hg) in gaseous or vapor form are entrained in the flue gas.
Coal fired power plants are required by law to limit emissions of sulfur and mercury carried in the flue gas exiting the plant stack. Effective in 2017, the Environmental Protection Agency (EPA)—Mercury and Air Toxics Standards (MATS) places tighter limits on allowable mercury emissions in the flue gas.
Sulfur can be removed from the flue gas using “Flue Gas Desulfurization” systems before the flue gas finally exits the power plant through the chimney stack. A wet FGD uses a special liquid consisting of water and slaked lime, atomized through spray nozzles in an absorber tower to bind with sulfur dioxide in the flue gas passing up through the absorber to form calcium sulfite and drain by gravity back into the liquid reservoir in the bottom. The captured sulfur (CaS) is removed through continuous blowdown near the bottom of the tank.
However, removing mercury can be more challenging because elemental mercury in its gaseous (vapor) state (Hg0) is inherently stable and therefore resists bonding to other compounds making it difficult to capture in this form. Therefore, chemical additives or reactants are often used to “oxidize” mercury into a more reactive state (Hg2+) so it more readily bonds to other compounds.
Mercury oxidant is a common chemical additive applied to the solid coal feed at the front end of a coal-fired combustion unit prior to combustion of the coal. Mercury oxidant reacts with relatively stable elemental mercury (Hg0) to make it a more reactive form of oxidized mercury (Hg2+) providing a higher affinity to binding with other chemicals.
Unfortunately, mercury oxidant is corrosive to carbon steel throughout the balance of plant when added in higher concentrations (above 100 parts per million (ppm)). FIG. 3 illustrates that higher levels of oxidation reactant create a higher incidence of corrosion. When mercury oxidant levels are above 100 ppm, corrosion becomes a serious side effect in the air heaters and flue gas ductwork in the power plant. Thus, it is of critical importance to keep the concentration of mercury oxidant well under 100 ppm and as low as possible to minimize the corrosive side effects.
Further, the oxidation reaction for mercury becomes less and less effective at higher levels of mercury oxidant on the fuel feed. Adding a secondary mode of mercury capture and removal, such as activated powdered carbon injection or organosulfides in the wFGD maintains mercury oxidant fuel feed additions below the corrosion threshold.
Another method of removing mercury from flue gas is to inject “activated carbon” into the flue gas. Activated carbon uses fine particles or powdered carbon (often referred to as PAC for Powdered Activated Carbon or ACI for Activated Carbon Injection) to chemically attract and bind with mercury. However, activated carbon as a “consumable” carries a high on-going operating cost as well as a high initial capital cost to purchase and install the infrastructure to inject the carbon and remove and dispose of the mercury.
One prior art system for removing mercury from flue gas is described in U.S. Pat. No. 8,632,742. Keiser describes a system for monitoring mercury concentration in a wFGD, correlating the mercury concentration to an amount of precipitant additive required to treat the flue gas and adjusting the amount of additive. However, this system fails to adequately address the influence of mercury oxidant added to the coal feed in the plant, fails to take into account actual stack mercury emissions and utilizes prior art precipitant technology. Additionally, Keiser's technology fails to address emission control issues encountered in power plants with multiple coal fired electric generating units.
Thus, there is a continuing need for a method of reducing mercury emissions in coal fired power plants with multiple coal fired electric generating units while reducing the consumption and operating costs of chemical reactants and their adverse corrosive side effects.